In the related art, regarding an automatic four-wheeled vehicle, a driving assistance system configured to control a traveling speed of a subject vehicle so as to secure an inter-vehicular distance between the subject vehicle and a preceding vehicle traveling in front of the subject vehicle has been suggested.
When detecting the preceding vehicle by means of a radar, detection performance of the radar is lowered under bad weathers such as heavy rain, dense fog and the like. As a result, it is determined that there is no preceding vehicle, so that speed reduction control is delayed and a passenger should resultantly operate a brake. In order to solve the problem, a technology of providing a rainfall amount detection unit configured to detect an amount of rainfall and stopping the inter-vehicular distance control when a detected rainfall amount value is equal to or greater than a predetermined value has been suggested (for example, refer to Patent Document 1).
Patent Document 1: Japanese Patent Application Publication No. H10-081156A
In the vicinity of a gateway of a tunnel, a flow of wind is complicated due to an influence of the wind moving in the tunnel. For this reason, since raindrops, fog and the like move intricately, the detection environments of the radar are temporarily deteriorated, so that the detection performance of the radar is temporarily lowered. As a result, a phenomenon that the preceding vehicle is late detected or is temporarily missed occurs.
When the speed of the subject vehicle is high, even the phenomenon for an extremely slight time period highly exerts a bad influence on a driving assistance. The detection delay and the temporary missing cause a rapid operation of an automatic brake to deteriorate a ride quality, cause a bad influence on a following vehicle, such as a harsh braking, and become an origination of a traffic jam.
The above phenomena become conspicuous when the preceding vehicle is a small-sized vehicle, particularly, a saddle-ridden vehicle.
Patent Document 1 does not disclose the temporary deterioration of the detection environments of the radar in the vicinity of the gateway of the tunnel.